1. Field of the Invention
Stabilization of soluble manganese and its reaction products
The present invention relates to the use of polyether polyamino methylene phosphonates to stabilize soluble manganese ions and their reaction products in desirable forms and reduced particle sizes. Manganese exists in various oxidation states from 1 through 7, with the oxidation states of 2 and 7 being the most stable, and therefore the predominant forms in nature. The present invention is intended to include stabilization of manganese ions for all of these oxidation states. Manganese ions are often found in well waters and cooling waters. Anionic species of carbonate, bicarbonate, sulfite, fluoride, chloride, sulfate, and so forth, and dissolved oxygen may be present in both waters. Oxygen reaction products of manganese and iron can collect on metal surfaces and accelerate corrosion and reduce heat transfer.
Oxidation leads to precipitation of dark brown or black hydrous oxides or hydroxides of the higher oxidation states of manganese which are very insoluble. Manganese hydroxide, MN(OH).sub.2, is especially a problem. When these precipitates remain suspended in the water, they cause objectionable discoloration known as "black water"; when they settle out, black deposits form which can block lines, or act as catalysts causing further manganese deposition. These deposits are very deleterious in textile and laundry operations as they interfere with dying processes and leave spots which are difficult to remove. They appear to increase the corrosion of copper. Manganese hydroxide, MN(OH).sub.2, while it is usually present as a colloidal suspension, when it does form a deposit, it readily becomes the site for the promotion of the deposit of other scaling materials, thereby accelerating scaling in general.
The polyether polyamino methylene phosphonates, when used in accordance with the method of the present invention, can keep the reaction products of manganese described above in colloidal/fine dispersed form rather than the normal flocculant, adherent species. The manganese thus remains soluble so that it will not form particles which will precipitate out of solution and form scale.
Stabilization of soluble iron and its reaction products
The present invention further relates to the use of polyether polyamino methylene phosphonates to stabilize soluble iron ion and its reaction products in desirable forms and reduced particle sizes. Ferrous and ferric ions are often found in well waters while cooling waters contain primarily the ferric species. Iron ions are often present as the result of rusting of the iron piping used to transport the water in a cooling system. Anionic species of carbonate, bicarbonate, sulfite, fluoride, chloride, sulfate, and so forth, and dissolved oxygen may be present in both waters. Oxygen reaction products of iron can collect on metal surfaces and accelerate corrosion and reduce heat transfer.
Oxidation leads to precipitation of brown or red oxides of the higher oxidation states of iron which are insoluble. When these precipitates remain suspended in the water, they cause objectionable discoloration known as "red water"; when they settle out, red deposits form which can block lines, or act as catalysts causing further iron reaction product deposition. These deposits are very deleterious in textile and laundry operations as they interfere with dying processes and leave spots which are difficult to remove. FE(OH).sub.2, while it is usually present as a colloidal suspension, when it does form a deposit, it readily becomes the site for the promotion of the deposit of other scaling materials, thereby accelerating scaling in general.
The polyether polyamino methylene phosphonates, when used in accordance with the method of the present invention, can keep the reaction products of iron described above in colloidal/fine dispersed form rather than the normal flocculant, adherent species. The iron thus remains soluble so that it will not form particles which will precipitate out of solution and form scale.
The present invention further relates to the use of polyether polyamino methylene phosphonates to stabilize soluble zinc ion and its reaction products in desirable forms and reduced particle sizes. Zinc ions are often found in well waters, while cooling waters can also contain zinc ions. Zn.sup.++ present in cooling waters is often derived from the zinc metal used in making the copper and brass alloys from which the piping used for transporting the cooling water is constructed. Anionic species of carbonate, bicarbonate, sulfite, fluoride, chloride, sulfate, and so forth, and dissolved oxygen may be present in both waters. Oxygen reaction products of zinc can collect on metal surfaces and accelerate corrosion and reduce heat transfer. Zinc hydroxide, Zn(OH).sub.2, has been found to be a particular problem. While it is usually present as a colloidal suspension, when it does form a deposit, it readily becomes the site for the promotion of the deposit of other scaling materials, thereby accelerating scaling in general.
The combination products of the zinc ions and the anionic species recited above can often settle out, creating deposits which can block lines, or act as catalysts causing further zinc reaction product deposition. These deposits can be very deleterious in textile and laundry operations as they interfere with dying processes and leave spots which are difficult to remove.
The polyether polyamino methylene phosphonates, when used in accordance with the method of the present invention, can keep the reaction products of zinc described above in colloidal/fine dispersed form rather than the normal flocculant, adherent species. The zinc thus remains soluble so that it will not form particles which will precipitate out of solution and form scale.
The stabilization methods described above are especially useful under conditions of high pH and high calcite concentrations, e.g., those found in cycled up cooling towers. Various industrial and commercial water-carrying systems are subject to zinc, iron and manganese deposit formation problems. These deposits form frequently in the tubes of heat exchangers and on other heat exchange surfaces, such as those in cooling towers. Particular systems or applications areas where severe conditions, especially high alkalinity, lead to exceptional buildup of zinc, iron and manganese deposits, in addition to cycled up cooling towers, include reverse osmosis systems, sugar refining evaporators, and certain types of gas scrubbers..
The polyether polyamino methylene phosphonates used in the methods of the present invention, are usually used in greater amounts than threshold inhibitors in the stabilization methods of the present invention, more closely resembling sequestering or chelating agents in amounts. The compositions of the present invention have dispersant properties as well and significantly reduce the adherency of any deposits which are formed, facilitating their easy removal.
Particular problems are encountered when attempting to prevent deposits of zinc, iron and manganese compounds under severe conditions, where conventional treatments do not provide complete control, and where high alkalinity causes precipitation of hydroxide salts due to increased insolubility. Conventional treatment can be used to inhibit zinc, iron and manganese deposits under normal conditions of alkalinity, e.g., up to 100 to 120 times calcite saturation, i.e., a water containing Ca.sup.2+ and CO.sup.2-.sub.3 present at 100 times (100 .times.) their solubility limit of calcium as calcite (calcite is the most common crystalline form of calcium carbonate). However, what is desired are inhibitors effective in greater than 150.times. water, especially in greater than 250.times. water, and more especially in greater than 300 .times. water, i.e., where the calcite ions can be prevented from precipitating as calcium carbonate scale using substoichiometric amounts of an inhibitor. The polyether phosphonate compositions used in the methods of the present invention are especially useful under severe conditions characterized by a calcite saturation level of 150.times. and above, especially 250.times. and above, and more especially 300.times. and above, as defined in the paragraph immediately below.
Another characteristic feature of the severe conditions under which the polyether phosphonate compositions used in the methods of the present invention are especially useful is high pH, i.e. a pH of 8.5 and higher, particularly a pH of 9 or 10 or even higher.
One of the particular advantages of the polyether phosphonate compositions used in the methods of the present invention is the exceptional calcium tolerance which they exhibit. Calcium tolerance is a measure of a chemical compound's ability to remain soluble in the presence of calcium ions (Ca.sup.2+). As pH increases, calcium tolerance decreases rapidly for many compounds which might be used to control zinc, iron and manganese deposits, and they precipitate with calcium at alkaline pH's, rendering them useless. While it is common practice to use an acid feed to the water of, e.g., a cooling tower system in order to lower pH and thus avoid the calcium tolerance problem, the danger to handlers which such acid feeding poses makes it all the more important to find inhibitors of zinc, iron and manganese deposits which operate at high pH's.
2. Brief Description of the Prior Art
Methods which have been used heretofore to remove manganese include those whereby the manganous ion is oxidized to insoluble higher oxides, hydrous oxides, or hydroxides, which precipitate and may be removed by coagulation and settling, filtration, or both. The oxidation has also been effected by raising the pH of the water to 8 or higher where naturally occurring dissolved oxygen or mechanical aeration brings about oxidation, or by the use of chlorine or permanganate. All of these methods, however, suffer from obvious disadvantages which limit their usefulness and effectiveness. For example, the use of a high pH to facilitate oxidation by dissolved oxygen is expensive and tends to cause scale deposition. Chlorine is only slightly more active than dissolved oxygen for oxidation of manganese and also requires pH elevation. Permanganate is expensive and imparts to the water an intense color that may be unacceptable.
One method for removing the manganese by precipitation and removal involves the addition of a salt of iron, copper, or cobalt and any compound yielding bisulfite ions in solution to the manganese-containing water. See Hatch - U.S. Pat. No. 3,349,031.
Soluble manganese ion and its reaction products have been stabilized in water systems using carboxylic acid/sulphonic acid copolymers. See Ralston - U.S. Pat. No. 4,552,665.
U.S. Pat. No. 4,080,375 discloses methylene phosphonates of amino-terminated oxyalkylates for use as scale inhibitors, but these compositions are not the same as those of the present invention, nor is there any suggestion that such compositions would be useful for stabilizing zinc, iron and manganese. U.S. Pat. No. 4,931,189 discloses aminomethylene phosphonates of the type used in the method of the present invention, but for inhibiting oil field scale formation involving a high brine environment susceptible to gypsum or barite scale formation. Such use in no way suggests the stabilization of zinc, iron and manganese described herein.
U.S. Pat. No. 4,783,267 discloses a method for stabilizing metal ions in recirculating water systems using 2-hydroxyphosphonacetic acid. The metal ions stabilized include iron, zinc, aluminum, and manganese. There is no suggestion, however, of use of the polyether phosphonates of the present invention.
The polyether polyamino methylene phosphonates of the type which are used to stabilize zinc, iron and manganese in the compositions of the present invention, are described in copending U.S. patent application Ser. No. 07/708,527, filed May 31, 1991. While their use for the control of calcium carbonate scale under severe conditions which include elevated pH and high calcium carbonate saturation levels, is described, there is no suggestion of their use to stabilize zinc, iron and manganese.